1. Field of the Invention
This invention relates to the use of pulse oximeters and methods for monitoring the occurrences of a patient""s sleep disorder breathing events. More particularly, this invention relates to a device and method for monitoring oxygen desaturations in a subject""s arterial blood flow (oximetry) as a result of sleep apnea events and other respiratory disturbances, and counting the number of these events that occur during a prescribed period.
2. Description of the Related Art
The diagnosis of a patient""s sleep disorders typically involves an analysis of the patient""s breathing disturbances during his or her sleep. These breathing disturbances are defined by the American Sleep Disorder Association and the American Sleep Apnea Association as being sleep xe2x80x9capneaxe2x80x9d if the disordered breathing is a pause that lasts ten or more seconds. They are further identified as: (1) Central Apneaxe2x80x94cessation of airflow (upper airwayxe2x80x94oral and nasal) and respiratory effort (amplitude of chest movement during breathing); (2) Obstructive Apneaxe2x80x94cessation of airflow with continuation of respiratory effort; (3) Hypopneaxe2x80x94decrease in airflow from baseline (typically one-third to one-half or more) with continuation of normal or decreased levels of respiratory effort; and (4) Cheyne-Stokes Breathingxe2x80x94a breathing pattern that characteristically shows cyclical breathing with progressively decreasing breathing to a shallow level followed by progressively increasing breathing in a decrescendo-crescendo pattern. During the shallow breathing period the decreases may be severe enough to be clear central hypopneas or apneas which last for several seconds. Such apnea events may occur hundreds of times during the sleep period and may lead to severe sleep disruptions and frequent awakenings.
The analysis and diagnosis of respiratory sleep pathologies currently involve a comprehensive testing method utilizing polysomnography (PSG). This procedure involves a full night testing in a medical sleep laboratory to monitor the temporal variations in the amplitude of the patient""s sleep-impacted, physiological parameters, including: a continuous measure of the level of oxygen saturation in the arterial blood flow (SpO2), heart rate, upper respiratory airflow, thorax and abdomen respiration efforts, electroencephalograms (EEG; electrical activity of the brain), electro-oculogram (EOG; electrical activity related to movement of the eyes), and electromyograms (EMG; electrical activity of a muscle). The PSG testing procedures are expensive as they are typically conducted in clinical settings by trained PSG technicians.
Current PSG equipment used for sleep testing share common, less-than-desirable features: (1) their use is expensive, since the equipment itself is expensive and a technician usually must be involved for its set-up and disconnection, plus the data collected must be subjectively analyzed by highly trained, sleep professionals, and (2) the recording devices require patients to be outfitted with tethered sensors for connection to bulky body monitors, computers or consoles such as a polygraph, thus, their size and weight does not allow the patient to be ambulatory, which can be essential for the evaluation of treatment efficiency and compliance.
Although PSG testing is the standard method establish for testing sleep disordered patients, there is strong evidence that measurements of the level of oxygen saturation in a subject""s arterial blood flow (SpO2) alone is useful for assessing a patient""s sleep-related, breathing disturbances. A compilation of several recent studies, which observed over seven thousand patients suspected or diagnosed with sleep apnea, reported typical oxygen levels decreases of anywhere form 2% to 4% for hypopneas (Note: The average oxygen saturation (profusion) baseline, measured at the finger of a subject, typically ranges from 92% to 98%). Some patients with central or obstructive apneas have been noted to experience arterial blood oxygen saturation decreases greater than 30%.
Oxygen desaturation events of ten seconds or longer duration and having a oxygen level decrease of 3% or more would appear to be a viable means for diagnosing the occurrence of a sleep hypopnea. See FIG. 1.
Such measurements of the level of oxygen saturation in a subject""s arterial blood flow (SpO2) are typically made with commercially-available pulse oximeters. Pulse oximetry has previously been described in many U.S. Patents, including U.S. Pat. Nos. 4,407,290, 4,266,554, 4,086,915, 3,998,550, and 3,704,706.
As blood is pulsed through the lungs by the heart action, a certain percentage of the deoxyhemoglobin, RHb, picks up oxygen so as to become oxyhemoglobin, HbO2. From the lungs, the blood passes through the arterial system until it reaches the capillaries at which point a portion of the HbO2 gives up its oxygen to support the life processes in the adjoining cells.
By medical definition, the oxygen saturation level is the percentage of HbO2 over the total hemoglobin; therefore, SpO2=HbO2/(RHb+HbO2). A person can lose consciousness or suffer permanent brain damage if the person""s oxygen saturation value falls to very low levels for extended periods of time. Because of the importance of the oxygen saturation value, it has been referred to as the fifth vital sign.
An oximeter determines the blood""s saturation value by analyzing the change in color of the blood. When radiant energy passes through a liquid, certain wavelengths may be selectively absorbed by particles which are dissolved therein. For a given path length that the light traverses through the liquid, Beer""s Law (the Beer-Lambert or Bouguer-Beer relation) indicates that the relative reduction in radiation power at a given wavelength is an inverse logarithmic function of the concentration of the solute in the liquid that absorbs that wavelength.
For a solution of oxygenated human hemoglobin, the absorption maximum is at a wavelength of about 640 nanometers (red), therefore, instruments that measure absorption at this wavelength are capable of delivering clinically useful information as to the oxyhemoglobin levels.
In general, noninvasive methods for measuring oxygen saturation in arterial blood utilize the relative difference between the electromagnetic radiation absorption coefficient of deoxyhemoglobin, RHh, and that of oxyhemoglobin, HbO2. It is well known that deoxyhemoglobin molecules absorb more red light than oxyhemoglobin molecules, and that absorption of infrared electromagnetic radiation is not affected by the presence of oxygen in the hemoglobin molecules. Thus, both RHb and HbO2 absorb electromagnetic radiation having a wavelength in the infrared region to approximately the same degree. However, in the visible region, the light absorption coefficient for RHb is quite different from the light absorption coefficient of HbO2 because RHb absorbs significantly more light in the visible spectrum than HbO2.
In the practice of pulse oximetry techniques, the oxygen saturation of hemoglobin in intravascular blood is determined by: (1) alternately illuminating a volume of intravascular blood with electromagnetic radiation of two or more selected wavelengths (e.g., a red wavelength and an infrared wavelength), (2) detecting the time-varying electromagnetic radiation intensity transmitted through or reflected back by the intravascular blood for each of the wavelengths, and (3) calculating oxygen saturation values for the patient""s blood by applying the Lambert-Beer transmittance law to the detected transmitted or reflected electromagnetic radiation intensities at the selected wavelengths.
Today""s conventional pulse oximeters generally are complex, table-top consoles or handheld devices. They are commercially available from many sources, including: Nonin (#8500 model), HealthDyne (#920M), Vitalog (#VX4), Novametrix (#510/511), Allied Healthcare (#3520), Criticare Systems (#5040), Lifecare International (SpotChek+), Datex-Ohmeda (#3900), Palco Labs (#300/340/400), Nellcor/EdenTec (#N-20/N-30), and Respironics (Cricket).
The accuracy of these devices is often plagued by the effects of motion artifacts which tend to cause erroneous oxygen desaturation measurements. However, the accuracy of such measurement may be improved by employing band-pass or digital filtering of the SpO2 signal.
Other filtering technologies, such as that described as xe2x80x9cMasimo Setxe2x80x9d, are employed by Ivy (#2000), Allegiance Oxi-Reader (#M2000-US), Quartz Medical (#Q-400) and others. These rely on a combination of a special probe sensor and an adaptive filtering technique to cancel motion artifacts. Meanwhile, another oximeter by SIMS BCI (#3404-000) uses serial autocorrelation technology to reduce the effects of motion artifacts.
Despite many advances in pulse oximetry, current oximeters are still bound by several major limitations, including: (1) their use of long tethers from the finger probe to the console which typically introduces artifact and signal inaccuracies, (2) the requirement for the presence of a trained professional to direct their set-up and use, (3) the typical restriction of their use to clinical or hospital environments, (4) their typical use with cumbersome console recording units that limit a patient""s mobility, (5) their output typically being provided in the form of temporal displays of a patient""s trends or average oxygen levels, and, most importantly for sleep medicine purposes, (6) they do not count and display, in real-time, the number of oxygen desaturations events experienced by a patient due to his/her disordered breathing events or disturbances during a prescribed time period.
Thus, an improved type of oximeter having the novel capability of providing counts of respiratory interruptions and disturbances would be a valuable contribution to sleep medicine. It would provide a less expensive alternative to PSG, and provide a relatively easy to use means for home monitoring of a patient""s sleep.
Additionally, with capability for unattended, ambulatory use, such an improved monitoring device would provide a preliminary screening alternative for assisting with the diagnosis of patients suffering from sleep disorders. For example, a device, that provides information as to which patients might benefit the most from complete PSG testing, could contribute greatly by effectively expanding the audience to whom PSG testing would be available.
Recognizing the need for a much simpler device and method for diagnosing sleep disorders, the present invention is generally directed to satisfying the needs set forth above and the problems identified with prior testing systems for assessing respiratory disturbances during sleep. The problems associated with the expense of PSG testing, its utilization only in clinical environments, the limited number of patients who are being assessed for sleep disorder breathing with PSG testing, and the non-ambulatory nature of such testing are resolved by the present invention.
In accordance with one preferred embodiment of the present invention, the foregoing need can be satisfied by providing a sleep disorder breathing event counter for counting the total number of sleep disordered breathing events experienced by the subject during a sleep period. This battered powered counter comprises: (1) an oxygen saturation level sensor for location at a prescribed site on the subject, with the sensor providing output data that quantifies the temporal variation in the subject""s oxygen saturation level at the prescribed site, (2) an oximetry conditioning and control module that controls the operation of the sensor and converts the sensor""s output signal to oxygen saturation data, (3) a miniature monitoring unit having a microprocessor with a memory device, a timer for use in time-stamping data, a display means and a recall switch, and (4) firmware integral to the microprocessor that directs the (i) sampling of the oxygen saturation data provided by the oxygen sensor conditioning and control module at prescribed time intervals, (ii) temporary storing of the oxygen saturation data in a designated area, buffer, of the memory device, (iii) the unit to analyze the temporarily stored data to detect an oxygen saturation decrease (desaturation) below a specified level (typically 3% or greater) from its baseline value, with such a decrease being the assumed definition of a disordered breathing event, (iv) storing time-stamped data corresponding to each of these events, (v) analyzing the stored, time-stamped data to count the total number of disordered breathing events that occurred during the subject""s sleep period, and (vi) the display means to display specified information pertaining to the counts in response to the actuation of the recall switch.
In another preferred embodiment, the present invention is seen to take the form of a method for counting the number of sleep disordered breathing events experienced by a subject within specified periods of sleep. For example, the number of disordered breathing events that occurred per hour during the sleep period. This method comprises the steps of: (1) locating an oxygen saturation level sensor at a prescribed site on the subject, the sensor providing output data that quantifies the temporal variation in the subject""s oxygen saturation level at the prescribed site, the sensor being connected to an oximetry conditioning and control module that controls the operation of the sensor and converts the sensor output data to oxygen saturation level data, the module being connected to a miniature monitoring unit having a microprocessor, a memory device, a timer for use in time-stamping data, a display means, a recall switch, and specified firmware for controlling the operation of the unit, (2) sampling the oxygen saturation level data at a prescribed frequency, (3) temporarily storing the sampled data in the memory device, (4) analyzing with a specified method the temporarily stored data to identify and count the occurrence of the subject""s disordered breathing events, (5) storing in the memory device the time of occurrence of each of the disordered breathing events, and (6) displaying specified information pertaining to the counts in response to the actuation of the recall switch.
Thus, there has been summarized above, rather broadly, the more important features of the present invention in order that the detailed description that follows may be better understood and appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject matter of any eventual claims to this invention.
In this respect, before explaining at least one embodiment of the present invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting.
It is therefore an object of the present invention to provide a device and method that can be used for counting in real-time the number of significant oxygen desaturation events that occur during a subject""s sleep.
It is another object of the present invention to provide a device and method that will advance the utility of pulse oximetry technology as applied to sleep medicine.
It is a further object of the present invention to provide a device and method for advancing the diagnostic capabilities of health practitioners in the field of sleep medicine.
It is yet another object of the present invention to provide a device and method that will further allow for the monitoring of the occurrence of a subject""s sleep disordered breathing events in the subject""s unattended, home environment.
These and other objects and advantages of the present invention will become is readily apparent as the invention is better understood by reference to the accompanying drawings and the detailed description that follows.